William Blake wrote the poem, but he did not write the music to which the hymn is sung. The hymn is often dismissed as a relic of imperialism, but I think Blake had a more profound religious message in mind when he wrote the poem.

I have read that Blake put many of his poems to music, in a folky rustic style, but lacking the skill to write down the music it has all been lost. Not much notice was taken of him in his time, probably not helped by the fact that he claimed to see angels in the trees. He was also an artist, and illustrated the books of his poetry, and originals of his work are worth a fortune.

A group from Kingston Ontario some time ago, in the 1960's or 1970's, set some of his poems to music, the best one being their version of his "Ah, Sunflower!" The public library in that city used to have a copy of this LP.